Solution and method for dissolving copper

ABSTRACT

AN ETCHANT SOLUTION IS DISCLOSED FOR USE IN DISSOLVING COPPER FROM COPPER-SURFACED ARTICLES, THE SOLUTION BEING OF THE AQUEOUS ALKALINE AMMONIACAL CHLORITE TYPE BUT MODIFIED BY THE INCLUSION OF A HALOGEN-SUBSTITUTED ACETIC ACID TO PROVIDE IMPROVEMENT IN THE CAPACITY FOR DISSOLVING COPPER AND UNUSUALLY UNIFORM RATE OF ETCHING THROUGHOUT THE USEFUL LIFE OF THE ETCHANT BATH. THE METHOD OF USING THE ETCHANT SOLUTION, AND A PROCESS FOR REJUVENATING AND RECYCLING IT, ARE ALSO DISCLOSED.

Feb. 20, 1973 l.. J. sLoMlNsKl Fr AL 3,717,521

SOLUTION AND METHOD FOR DISSOLVNG COPPER Original Filed Sept. 24, 1970 0 0. o amNlw/swv -alvu H013 INVENTORS L EO J. SLOMINSKI ADELA LANDAU ARTHUR J. SIEGMUND ATTORNEYS United States Patent O U.S. Cl. 156-19 14 Claims ABSTRACT OF THE DISCLOSURE An etchant solution is disclosed for use in dissolving copper from copper-surfaced articles, the solution being of the aqueous alkaline ammoniacal chlorite type but modified by the inclusion of a halogen-substituted acetic acid to provide improvement in the capacity for dissolving copper and unusually uniform rate of etching throughout the useful life of the etchant bath. The method of using the etchant solution, and a process for rejuvenating and recycling it, are also disclosed.

This is a continuation of application Ser. No. 75,106, filed Sept. 24, 1970, now abandoned.

This invention is directed to etchant solutions for dissolving copper on the surface of articles, and to methods of using such solutions. More particularly it is directed to solutions and methods for dissolving copper films or claddings carried by various substrates which themselves may be of metal other than copper, or a non-conductive, e.g. plastic, material. The etchant solution of the invention is of particular utility in the etching of electronic printed circuitboards.

By way of brief review, the commercially important etchant solutions in common use today to dissolve copper include the following. Aqueous solutions of ferrie chloride are extensively employed as they provide a fast and economical etchant. However, this system lacks selectivity in respect to copper, in that it attacks other substrate metals, such as steel or tin-lead alloys and nickel commonly used as plated resists in the manufacture of printed circuitboards. Cupric chloride is almost as fast as ferric chloride, and is essentially as economical if it is regenerated. Again, however, this system attacks plated resists. Ammonium persulfate has also been used extensively but is slow, unless catalyzed by mercury in which case a waste disposal problem is presented. Mixtures of chromic and sulfuric acid are also used as copper etchants and are intermediate in speed and capacity to the foregoing chloride and persulfate systems. But the presence of the chromic ion in this system is again particularly troublesome with respect to satisfactory waste disposal. Undercutting is also a problem here. Ammoniacal chlorite etchants of the type disclosed in U.S. Pats. Nos. 3,231,503 and 3,466,208, provide a fast rate of etching and relatively high copper-dissolving capacity (l-12 ounces per gallon) in commercial practice. They are at the same time quite selective to copper providing minimum interference with plated metal resists and produce no significant undercutting. The ammoniacal chlorite etchant solutions also minimize waste disposal problems since the copper can be easily precipitated and reclaimed in commercially recoverable form. The only major disadvantage of the ammoniacal chlorite etchants has been the need for rather close correlation and control of the solution temperature and time of exposure of the copper surfaced articles to the solution.

3,7 l 7,52 l Patented Feb. 20, 1973 ICC For example, it has been found necessary in the use of the prior ammoniacal chlorite etchant solutions to start the etching operation at a solution temperature of around F., and subsequently increase this progressively to around F. over the useful life of the bath. This represents a practical upper temperature limit as loss of ammonia through volatilization becomes excessive above that limit. Consequently the practice has been to adjust the time of contact or exposure of the copper surfaced article to the solution during the life of the bath in order to compensate for progressive loss of etching speed with increase in dissolved copper in solution. Such a procedure obviously requires close control and constant supervisory attention.

It has been found that unexpected improvement in etching properties and procedure are obtained by incorporating the above-mentioned halogen-substituted acetic acid in the ammoniacal chlorite bath. Two of the more significant improvements in the novel etchant solutions of the invention are an aproximate doubling of the copper capacity of the system before it must be replaced or rejuvenated, and a remarkably uniform rate of etching throughout the useful life of the bath without temperature adjustment.

It has been known for a long time that an ammoniacal solution of trichloroacetic acid itself will attack metallic copper in powder form; cf. Doughty, Hydrolysis of Organic Halides and the Corrosion of Metals, 39 J.A.C.S. 290 (1917); Doughty and Freeman, Reaction of Trichloroacetic Acid with Copper, 44 J.A.C.S. 638 (1921); same, The Use of Silver Cathodes in Electro-deposition of Copper, 44 J.A.C.S. 702. However the etch rate on copper of a freshly prepared ammoniacal solution of trichloroacetic acid (hereinafter called TCA) is found to be unacceptably low and of no interest for practical industrial use. But a surprising change is found to occur after a sufficient period of time has elapsed to bring the dissolved copper content up to a level of aproximately 6 ounces per gallon. The inference drawn from this is that TCA is' not too effective as a primary oxidant and that, more likely, the effective copper dissolving reaction is the result of oxidation of metallic copper by divalent copper. Presumptively TCA thus serves chiefly as a secondary oxidant, oxidizing monovalent copper to divalent; or possibly it acts only as a catalyst assisting in air oxidation. Once the etch rate of this system reacts a maximum, it thereafter remains quite stable until the concentration of dissolved copper in the solution dictates replacement or rejuvenation of the 'bath to avoid precipitation.

It is the basic discovery of the present invention that the ammoniacal chlorite and TCA etchant systems will thus act in complementary manner when combined, without destroying or adversely affecting the etching properties of either of them. As will appear more fully hereinafter, this combination of ammoniacal chlorite and 'PCA is substantially unique in that combinations of ammoniacal chlorite with oxidants other than halogen-substituted acetic acids adversely affect the operation of the chlorite system, and vice versa. The uniqueness of the combination extends to the monoand dichloro homologs of TCA, as well as the mono, diand tri-bromo-substituted acetic acids.

It is accordingly an object of this invention to extend the useful copper-dissolving capacity of the ammoniacal chlorite system, and to provide a system which has a relatively constant, adequately high etch rate at constant bath temperature throughout its useful life. In addition, it is an object to provide a system which is easily regenerable and from which it is possible to recover the dissolved copper economically and without waste disposal problems. It is also an object to provide an etchant solution which produces a minimum of undercut in respect to masked portions of copper surface being etched. Gther objects of the invention will appear as the description proceeds.

The invention is illustrated by the examples given below and by the graphs shown in the accompanying drawing which is a graph comparing etch rate in mills/ min. against ounces of copper dissolved in the solution for different solutions as represented by the several curves.

Example l-Prior art A commercial spray etcher (Chem-Cut, Model 502) was charged with v60 gallons of etchant solution of the following composition:

Sodium chlorite-0.375 molar Ammonium chloride-1.0 ymolar Ammonium bicarbonate-0.75 molar Ammonium hydroxide-3.4 molar Waterto make up 60 gallons The foregoing solution is a commercial copper etchant bath used extensively heretofore in the manufacture of printed circuitboards, and corresponds very closely with the system disclosed in prior U.S. Pat. No. 3,466,208 (see Table l, Ex. No. This solution was preheated to about 90 F. and then a regular production run of printed circuitboards with a plated tin-lead resist was etched. The heat of reaction raised the temperature to around 100 F. as the etching progressed, and the conveyor speed had to be increased initially as this occurred to prevent over-etching. With further etching leading to progressive exhaustion of the solution, the temperature was increased incrementally to maintain a relatively constant etch rate, up to the limiting temperature of 120 F. which represents a practical limit above which excessive volatilization of ammonia occurs. During this period, the conveyor speed was decreased correspondingly to compensate for decreasing etch rate until it became impractically slow, at which time further use of the bath Was discontinued. The total dissolved copper at this point was 10.4 ounces per gallon.

The plot of etch rate against dissolved copper content for this system is shown as Curve A in the accompanying drawing. In this system, the start-up temperature of the new bath must be kept relatively low to avoid an excessive etch rate which, as noted however, rapidly decreases With increase in dissolved copper content if the bath temperature is held constant. As a practical compromise to produce a reasonably constant etch rate therefore the bath is started at the lower temperature and the temperature is allowed to increase through heat of reaction during the initial operation, up to about 2 oz. of copper per gallon. At this point it becomes necessary to supply external heat to maintain an acceptable rate of etch as the Solution becomes more and more saturated with copper. As is apparent from Curve A, however, the compromise is only partially effective, in that the rate of etching varies appreciably over the life of the bath. Even to obtain this compromise rate it is necessary to adjust both the bath temperature and time of exposure of the articles during the process.

-Exarnple 2-Prior art Again a commercial spray etcher was charged with a bath of the following composition:

TCA-1.0 molar Ammonium chloride-2.4 molar Ammonium hydroxide- 6.0 molar Water The solution Was heated, as prepared, to 120 A regular production run of printed circuitboards with a 4 plated tin-lead resist was put through, and a determination made of the etch rate against dissolved copper content in the etchant solution.

Although there is some suggestion in the earlier work on TCA copper systems mentioned above that that the reaction is quite vigorous with copper powder, the foregoing test showed an extremely slow initial rate of etch on solid copper sheet or foil. Surprisingly however, this increased as the dissolved copper content increased. It is conjectured, therefore, that the primary oxidizing agent in the system is probably the cupric ion, as reported in U.S. Pat. No. 3,231,503, which reacts with metallic copper to form the cuprous ion, and TCA then acts as a secondary oxidant, converting the resultant cuprous to cupric ions.

In order to maintain the eiciency of etch rate in this solution, it is necessary to make periodic additions of ammonium hydroxide to -keep the pH above 8.5. A plot of etch rate versus dissolved copper content for this system is represented by Curve B on the graph of the accompanying drawing. The low starting rate of etch is evident, after which it increases generally in proportion to dissolved copper content up to about l0 oz. per gallon where it levels off and then decreases slightly at copper concentrations around 20 oz. per gallon. Sludging or precipitation begins to develop at this point which interferes with proper etching of the circuitboards, even though the etch rate is still relatively high.

Example S-Invention Again a commercial spray etcher was charged with' a solution whose composition in this case was as follows:

Sodium chlorite0.2 molar Trichloracetic acid-1;() molar Ammonium chloride-2.4 molar Ammonium hydroxide-6.0 molar Water The system was operated at a constant temperature of F. and the conveyor speed was also kept constant. The performance curve, Curve lC on the accompanying graph, shows that a virtually constant etch rate is obtained over a range of dissolved copper content of zero to in excess of 20 oz. per gallon. While the etch rate of the invention system (Curve C) is diminished only slightly when the dissolved copper content reaches 20 ounces per gallon, it is necessary as in Example 2 to replace or regenerate the bath to avoid problems from sludging due to copper precipitation.

Regeneration can be eected by lowering the pH of the bath, preferably with hydrochloric acid, to precipitate the copper. Any acid is satisfactory for this when the anions introduced are not incompatible with the etching system. The precipitate can then be separated by filtration, decantation or centrifugation and the resultant supernatant solutions refortied or regenerated by the addition of TCA, ammonium hydroxide and, depending on the extent of copper precipitated, sodium chloride. Since much of the decomposition products resulting from the etching operation are carried down by the precipitate during the regeneration process, there is no continuing accumulation of `lay-products and regeneration can be contained virtually indefinitely. An example of this follows.

Example 4-Regeneration A l liter solution was prepared having a composition the same as that of Example 3 above. This solution was used to etch copper circuitboards in the same manner as described in Example 3 until the concentration of dissolved copper reached a level of 16 ounces per gallon. At this point, hydrochloric acid was added to bring the pH of the solution down to 6.5 and the resulting copper precipitate was filtered oif. The supernatant solution measured 800 ml., and the copper content was analyzed at 10 ounc-es per gallon. This was refortified with 100 ml. of the following solution:

TCA-820 grams Aqua ammonia-385 ml. Water-l65 ml.

After the addition of the concentrate, somewhat more ammonia was added to bring the pH to 8.5 and the volume was adjusted to 1 liter with water. The regenerated solution was used again to etch copper from circuitboards and the regeneration process repeated each time when the dissolved copper solution reached about 16-20 ounces per gallon. After five such cycles the bath continued to function entirely satisfactorily.

The concentrations of sodium chloride, TCA and ammonium salt in the etchant solution are not critical from an operability standpoint, although there are certain concentrations and ratios between components which are preferred for economic reasons or reasons of commercial practicability. For example, the indicated initial chlorite concentration of 0.2 molar is selected to provide an early etching rate (roughly 1.0 mil/min.) to match the rate obtained during balance of the life of the bath when operated at 120 F. At higher initial concentrations of sodium chloride, such as 0.375 molar suggested in my earlier patent, No. 3,466,208, the etch rate is almost 2 mils/min. at 120 F., and the heat of reaction is so great that cooling becomes a problem. Except for this practical consideration, however, the upper limit for chlorite would be saturation, especially when operating at a lower system temperature. As a lower limit for the chlorite, some benefit is obtained at a level of 0.05 molar but generally 0.1 molar is considered necessary for practical purposes.

TCA can also be used in amounts up to saturation, but little benefit is derived at levels above one molar, since this will dissolve about as much copper as the system will hold in solution without encountering the sludging problem mentioned above.

The ammonium chloride content should be at least equivalent to the dissolved copper, up to about 3 molar. Since TCA will release chloride as it is being reduced during the etching process, the final solution will be about 6 normal in respect to chloride content. In addition to ammonium chloride, the following ammonium salts are also found to be useful as anion acceptors: nitrate, sulfate, carbonate, acetate, sulfamate, benzenesulfonate, phosphate and pyrophosphate.

It is not possible to add enough ammonia to the initial solution to complex all the copper the system will dissolve. Therefore, ammonia is replenished periodically to maintain the system pH above 8.5, with a preferred upper limit of about 1l. This is best accomplished by an automatic monitoring device, with either aqueous or anhydrous ammonia being added in response to a pH change.

The temperature of the etchant solution can be anywhere from ambient to about 130 F. At this upper level, ammonia is volatilized and lost so rapidly that it is preferred to limit the operating temperature to about 120 F. for maximum etch rate with reasonable economy.

Chloroacetic acids other than TCA are also operative, as shown by the following examples.

Example 5 The procedure of Example 3 above was duplicated, with the exception of the substitution of 1 molar dichloroacetic acid for TCA. The results are plotted as Curve D on the accompanying graph, from which it will be seen that although the average etch rate is somewhat lower than for TCA, the system is still effective as an etchant.

Example 6 Again the procedure of Example 3 was duplicated, except for the substitution of 1 molar monochloroacetic acid for TCA. The results are plotted as Curve E in the accompanying graph from which it will be seen that the system is still effective although total copper capacity is further reduced relative to the dichloro and trichloro homologs.

Example 7 The procedure of Example 3 is duplicated except for the substitution of l molar tribromoacetic acid for TCA. Good etching is obtained, but the bromo-hololog is much more expensive and not readily available. The same is true of the monoand di-bromoacetic acids.

What is claimed is:

1. An aqueous solution for dissolving metallic copper, which consists essentially, apart from water, of

(a) sodium chlorite at a concentration of from 0.05

molar to saturation;

(b) ammonium hydroxide sufiicient to maintain the pH of the solution alkaline;

(c) an ammonium salt in concentration up to 6 normal to complex dissolved copper, the anion of said salt being selected from the group consisting of chloride, nitrate, sulfate, carbonate, acetate, sulfamate, benzene-sulfonate, phosphate, phyrophospate and combinations thereof; and

(d) a supplemental oxidant having the general formula XnCH(3 n)-COOM where X is chloro or bromo, n is an integer from 1 to 3, and M is hydrogen, ammonium, sodium or potassium, said oxidant being present at a concentration of 0.1 molar to saturation.

2. An aqueous solution for dissolving metallic copper as defined in claim 1, wherein the amount of ammonium hydroxide present is sufiicient to maintain the pH of the solution from 8.5 to 11.

3. An aqueous solution as defined in claim 1, wherein said supplemental oxidant is trichloroacetic acid.

4. An aqueous solution as defined in claim 3, wherein said trichloroacetic acid is approximately 1.0 molar, said sodium chlorite is approximately 0.2 molar and said arnmonium salt is approximately 2.4 molar ammonium chloride.

5. An aqueous solution as defined in claim 1, wherein said supplemental oxidant is dichloroacetic acid.

6. An aqueous solution as defined in claim 5, wherein said dichloroacetic acid is approximately 1.0 molar, said sodium chlorite is approximately 0.2 molar, and said ammonium salt is approximately 2.4 molar ammonium chloride.

7. An aqueous solution as defined in claim 1, wherein said supplemental oxidant is monochloroacetic acid.

8. An aqueous solution as defined in claim 7, wherein said monochloroacetic acid is approximately 1.0 molar, said sodium chlorite is approximately 0.2 molar, and said ammonium salt is approximately 2.4 molar ammonium chloride.

9. A process of dissolving metallic copper from an article having a copper surface, which comprises:

(a) providing an etchant solution of the composition defined in claim 1;

(b) contacting the copper surface of said article with said solution for a period of time sufficient to dissolve the desired amount of copper from such surface;

(c) periodically adding ammonium hydroxide to the etchant solution to maintain the solution alkaline.

10. A process of dissolving metallic copper from an article having a copper surface as defined in claim 9, wherein ammonium hydroxide is periodically added to the etchant solution to maintain the pH between 8.5 and 11.

11. A process defined in claim 9, wherein the temperature of the etchant solution is maintained at approximately F.

12. The method of etching copper from the surface of a non-conductive substrate in the manufacture of electronic printed circuitboards, which comprises bringing the copper surface of the printed circuitboard into contact with an aqueous etchant solution consisting essentially, apart from water, of 0.2 molar sodium chlorite, ammonium hydroxide sucient to maintain the pH of the solution alkaline, from about 2.4 to 6.0 molar arnmonium chloride and about 1.0 molar trichloroacetic acid; maintaining the temperature of said etchant solution between 60 F. and 130 F. and contacting said copper surface with said etchant solution for a period of time suticient to remove the desired amount of copper from said surface.

13. The method of etching copper as defined in claim 12, wherein the solution contains ammonium hydroxide suicient to maintain the pH at from 8.5 to 11.

14. The method of etching as defined in claim 12, wherein said etchant solution is periodically regenerated by withdrawing at least a portion thereof to a storage tank, acidifying said portion to reduce the pH to precipitate copper, separating the supernatant solution from said precipitate, refortifying said supernatant solution by the addition olf trich-loroalcetic acid, and ammonia sucient to raise the pH of the solution to at least 8.5, then recycling this refortied portion of the etchant solution with the remainder thereof.

References Cited UNITED STATES PATENTS 3.287,191 11/1966 Borth 156-3 3,407,141 10/ 1968 Banish et al. 252-794 3,463,733 8/ 1969 Achenbach 252-79.4

3,600,244 8/ 1971 Wegener 156-19 JACOB H. STEINBERG, Primary Examiner U.S. Cl. X.R. 

